Hammer drills are known in which rotation of toothed surfaces against each other causes a hammering action. Also, in U.S. Pat. Nos. 3,149,681 and 3,133,602, rotary impact hammers with a ball on tooth engagement provide for a hammering action only in one direction of rotation. A ball on tooth engagement also tends to wear a groove in the tooth, which tends to create a wide contact area between ball and tooth. Together with the immobility of the tooth surface, the wide contact area increases friction losses and heating of the tool.
The present invention provides a hammer drill with rolling contact at the contact surfaces for transmission of axial force between a drive shaft and hammer shaft. In the case of ball bearings, point contact is obtained. In the case of roller bearings, line contact is obtained. The area of contact is thus close to zero as opposed to a relatively large area in engagement systems using toothed surfaces. Use of point or line contact reduces heat generation and reduces energy loss due to friction.
In some prior art products, a release clutch is used to release torque when pressure is critically increased and to prevent engagement parts from shear. In the case of a hammer drill with rolling contact, relatively low torque generators may be used where the torque does not exceed shearing stresses. The hammer drill of the present invention does not require the release clutch because it provides its function by rolling friction. When torque increases, the rotating bearing elements in the drive assembly are pushing the rotating bearing elements in the hammer assembly, thus separating the hammer assembly from the drive assembly and releasing the torque. This repetitive action also generates a hammering effect. The contact points between the rotating bearing elements are between 0 and 90 degrees to the tool axis. This offset makes the shearing component of the reaction force to rotate the rotating bearing elements inside the cavities and its axial component makes rotating bearing elements climb on each other.
To provide easier assembly and better interaction control between driver half and hammer half of the hammer drill, the bearing holders are provided by a plate with cavities backed up with a back plate. Rotating bearing elements, preferably balls, inserted into the cavities are exposed above the front surface of the cavity plate. The rotating bearing elements are prevented from axial motion in relation to the bearing holder, but are allowed to rotate freely within the cavities of the bearing holder.
The balls of one bearing holder may be fewer in number than the balls of the other bearing holder, and the balls of one bearing holder may be radially offset in relation to the balls of the other bearing holder. An on-off switch is also provided to turn the hammering action on and off.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.